This is sort of a “highlights of Cologne” run. Over 13k, you’ll enjoy the linear park of the Inner Green-Belt (Innerer Grüngürtel), a section along the Rhine, and then some of the major sights of Historic Cologne.
Jump on anywhere, a good place to start is at the southern end of the Grüngürtel. Run the 4 km to the end (click here for detailed description). Cross busy Innere Kanalstraße, and run along the historic Neusser Wall to by Fort X. Head down to the river and turn right on Aduchtstraße, crossing Riehler and Worringer Str. Then head south along the Rhine path to the Rheingarten, turning right at the Fish Market onto Lintgasse. Pass by the Groß St. Martin, through Marsplatz, observing City Hall, the old Jewish Quarter, the Wallraf-Richartz Museum (oldest museum in the city), and the Perfume Museum. Left on Quatermarkt, and right on Schildergasse, passing by a major shopping area. Left on Fleischmengergasse, where the road then turns into Kleiner Griechenmarkt, crossing the 55 to the Eifelplatz rotary and the Volkspark. Here, you can do a a ~1.3 km tour of the major paths of this lovely park, then heading up the Eifelwall to the start of the run.
Add-On: You can continue through the linear park for a couple of km, through Lis-Böhle-Park and Johannes-Giesberts-Park. Or, do more along the Rhine.
Shorter Option: This whole run is within the inner part of the city, well serviced by transport, so you can stop at a station to return to the start!